As is well known, an ion guiding device is an indispensable key component for a mass spectrometer, and the performance of the ion guiding device greatly affects the whole mass spectrometer in terms of many properties such as sensitivity, mass range and scanning speed. In view of various ion guiding devices used in the current main commercial mass spectrometers, a quadrupole rod and a multipole rod (generally, a hexapole rod or an octupole rod) typically are the most common structures. As an ion guiding device, a quadrupole rod has the advantage of better ion beam compression effects, thus facilitating efficient introduction of ions to a next-stage ion optical device. However, a quadrupole rod has a smaller ion acceptance area and a lower ion transmission efficiency when compared with a multipole rod. Additionally, a quadrupole rod and a multipole rod generally have a low operating gas pressure to ensure that an RF field generated by a radio-frequency voltage can effectively confine ions. Meanwhile, the axial transmission of ions can be achieved only by initial kinetic energy or gas flow driving due to the absence of an axial driving electric field, thus the ions can dwell therein for a relatively longer period of time, which affects the analysis speed of an instrument.
In an U.S. Pat. No. 6,107,628, Richard D. Smith et al. disclose an ion funnel technique, wherein a series of ring electrodes with inner diameters gradually reduced are used and then stacked in an axial direction to form a funnel structure with a tapered opening. Because radio-frequency voltages of opposite phases are applied among adjacent ring electrodes and the electrodes are in series connection via resistors to form an axial voltage dividing structure, a radial multipole field and a axial electric field are formed in the ion funnel structure to achieve axial transmission and radial compression of ions. The apparatus has the advantages of the ion compression function of a quadrupole rod and the large ion acceptance area and high ion transmission efficiency of a multipole rod, and can operate at an extremely high gas pressure. However, this technical solution employs an stacked structure of the ring electrodes such that the electrodes are segmented axially, thus an axial voltage dividing design is required to achieve the axial driving of ions, resulting in that the apparatus has a relatively complicated structure and circuit connection and cannot be conveniently machined. Additionally, the apparatus has a poor resistance to sample contamination.
In another U.S. Pat. No. 8,299,443B1, Alexander A. Shvartsburg et al. propose a planar ion funnel device, wherein a planar structure design as well as an extremely small electrode size and electrode gap (<200 um) are employed based on the above ion funnel device to increase the operating gas pressure and discharge voltage of the ion funnel. However, the basic operating principle and design concept of this planar ion funnel device are not greatly different from those of the above ion funnel device.
Additionally, in an U.S. Pat. No. 8,835,839B1, Gordon A. Anderson et al. propose a structure for lossless ion manipulation, wherein the structure consists of electrode arrays adhered on two surfaces, thus various ion manipulations can be achieved in the structure by applying RF voltage and DC voltage. The flexible expansibility of the structure greatly depends on a set of parallel planar electrode structures. Additionally, it is difficult for the structure to achieve the ion compression effects.
In an U.S. Pat. No. 5,847,386, Bruce A. Thomson et al. propose an ion guiding structure consisting of rod electrodes, wherein an axial driving electric field is established by changing the cross section area of the rod electrodes and the head-to-tail spacing between the rod electrodes, and also propose the use of auxiliary electrodes to adjust the axial electric potential distribution. However, in the structure, the establishment of the axial driving electric field requires a change in the section of the rod electrodes, and involves changing the head-to-tail spacing between the rod electrodes in two directions, i.e. an inlet section and an outlet section forming the structure are both changed in two directions, and the structure additionally depends on the auxiliary electrodes, which causes this structure to be relatively complicated.
Similarly, in another U.S. Pat. No. 7,868,289B2, Lisa Cousins et al. propose an ion guiding device consisting of parallel rod electrodes with rectangular cross-sections, wherein the cross-sections of the rod electrodes are gradually reduced along the length thereof, and the ion guiding device is surrounded by an additional cylindrical electrode, thus establishing an axial driving electric field. Likewise, the structure is relatively complicated and depends on the cylindrical electrode.
Additionally, in an U.S. Pat. No. 8,193,489B2, James L. Bertsch et al. propose an ion guiding and compression device which allows a multipole rod to be gradually evolved into a plurality of quadrupole rods, but, in the device, it is necessary to apply a DC voltage drop along a length direction thereof onto rod electrodes so as to establish an axial driving electric field.
Meanwhile, in an U.S. Pat. No. 6,891,157B2, Robert Harold Bateman et al. also propose an ion transmission structure consisting of plate electrodes placed on a plane. The device is characterized in that a complicated ion transmission path can be achieved by combining different plate electrodes, thus reducing the noise resulting from neutral molecules. However, a concept is not provided in this patent that an penetration voltage gradient is produced in an axial direction by applying a voltage gradient in a direction perpendicular to the axial direction so as to drive axial transmission of ions. Additionally, the device is also limited in that the device greatly depends on plate electrodes, thus a larger capacitance occurs between the electrodes of the device, which causes a greater power consumption when ions are confined using a RF power source.
In summary, the electrode structures and circuit connections in the existing ion guiding and compression techniques are relatively complicated and cannot be conveniently machined. Accordingly, it is currently a research subject of intense interest to use a simple structure to achieve highly efficient ion transmission and compression.